1. Field of the Invention
The present invention relates to a differential signal transmission cable, more particularly, to a differential signal transmission cable for transmitting high speed digital signals corresponding to 10 Gbps over a distance of several meters to several tens of meters with less signal waveform distortion.
2. Related Art
In servers, routers and storage associated equipments for processing high speed digital signals of several Gbps or more, differential signal transmission is used for signal transmission between devices or between boards in the same device, and a differential signal transmission cable is used as transmission medium.
The “differential signal transmission” is a signal transmission of transmitting two kinds of signals, in which a phase of one signal is inverted by 180 degrees from a phase of another signal, through a pair of two conductor wires respectively, and taking out a difference between the two signals at a receiving end side.
Since electric current flown through one of the two conductor wires and electric current flown through another one of the two conductor wires are flown in directions opposite to each other, an electromagnetic wave emitted from the differential signal transmission cable which serves as a transmission line is small. Further, since extraneous noises equally superpose on the two conductor wires, the extraneous noises are canceled (offset) by taking out the difference at the receiving end side, so that adverse influences by the extraneous noise can be removed. For these reasons, the differential signal transmission has been often used for high speed signals.
As representative differential signal transmission cable, a twisted-pair cable has been known. In the twisted-pair cable, two insulated electric wires each of which has a conductor wire coated with an insulating member are twisted as one pair.
The twisted-pair cable is inexpensive and excellent in balancing characteristics. Further, the twisted-pair cable can be easily bent. Therefore, the twisted-pair cable has been used broadly. However, since the twisted-pair cable has no conductor corresponding to a ground, the twisted-pair cable is easily affected by a metal member located in vicinity of the twisted-pair cable, so that characteristic impedance of the twisted-pair cable is not stable. Further, in the twisted-pair cable, a signal waveform is easily distorted in a high frequency band of several GHz. Therefore, it is difficult to employ the twisted-pair cable for the high speed signal transmission of several Gbps.
As to a shielded twisted-pair cable in which a shield is provided at an outer side of the twisted-pair cable, such a shielded twisted-pair cable has been already proposed as LAN cable. A tolerance for the extraneous noise is improved by an effect of shield. However, as for the twisted-pair cable, since the two conductors are twisted as one pair, attenuation of the signal is large. In a system using the shielded twisted-pair cable, an electric power required in signal processing for compensating the attenuation of the signal is increased (six times to ten times of the electric power required in a case of using a twinax cable to be described later), so that a power consumption is large.
On the other hand, the twinax cable in which two insulated electric wires are disposed in parallel without being twisted, and coated with a shield conductor has been used broadly. The “twinax cable” is also called as “twin-axial cable” or “twin coaxial cable”. In the twinax cable, the two insulated electric wires are disposed in parallel without being twisted, so that there is little difference in physical length between the two conductor wires, compared with the twisted-pair cable. In addition, since the shield conductor are disposed to cover the two insulated electric wires, even if the metal member is installed in vicinity of the twinax cable, the characteristic impedance of the twinax cable will not become unstable, and the noise resistant property is high.
The twinax cable has been used for the high speed signal transmission of several Gbps or more. There are various type of twinax cable, for example, a twinax cable using a tape with a conductor as a shield conductor, a twinax cable using a braided wire as a shield conductor, and a twinax cable using a drain wire together with a shield conductor.
FIG. 12 shows a cross-sectional view of a first example of conventional twinax cables. As shown in FIG. 12, in the first example of the conventional twinax cables, two signal transmission conductor wires 1201, 1204 are insulated by insulating members 1202, 1205, respectively to provide two insulated electric wires 1203, 1206, and a shield conductor 1207 comprising a metal foil tape in which aluminum or the like is adhered to a polyethylene tape is wound around the two insulated electric wires 1203, 1206. A drain wire 1208 is lengthwise provided between the shield conductor 1207 and the insulated electric wires 1203, 1206 to contact a conducting plane of the shield conductor 1207, so as to ground the shield conductor 1207. An outer surface of the shield conductor 1207 is jacketed with a jacket 1209 so as to protect a cable interior. The shield conductor 1207 is electrically connected to a printed circuit board (not shown) via the drain wire 1208 which is in contact with the shield conductor 1207.
FIG. 13 shows a cross-sectional view of a second example of conventional twinax cables, which is disclosed by Japanese Patent Laid-Open No. 2004-79439 (JP-A 2004-79439). As shown in FIG. 13, in the twinax cable of the second example, two conductor wires 1301, 1304 are insulated by insulating members 1302, 1305, respectively to provide two insulated electric wires 1303, 1306, and a shield conductor 1307 is wound around the two insulated electric wires 1303, 1306. A drain wire 1308 is lengthwise provided between the shield conductor 1307 and the insulated electric wires 1303, 1306 to contact a conducting plane of the shield conductor 1307, so as to ground the shield conductor 1307. The shield conductor 1307 is jacketed with a jacket (not shown), similarly to the twinax cable of FIG. 12. However, in the second example, the drain wire 1308 having a non-circular cross section is used so as to reduce displacement (location gap) of the drain wire 1308. This twinax cable is configured based on an expectation that a stress acting between the insulated electric wires 1303, 1306 and the drain wire 1308 may be dispersed, thereby suppressing collapse of the insulating members 1302, 1305.
FIG. 14 shows a cross-sectional view of a third example of conventional twinax cables, which is disclosed by Japanese Patent Laid-Open No. 2003-297154 (JP-A 2003-297154). As shown in FIG. 14, in the twinax cable of the third example, two conductor wires 1401, 1404 are insulated by an insulating member 1402, and a drain wire 1408 is lengthwise provided on the insulating member 1402. A shield conductor 1407 is wound around an outer periphery of the insulating member 1402 as well as the drain wire 1408. The shield conductor 1407 is jacketed with a jacket 1409. In the third example, so as to solve the problem of the location gap of the drain wire 1408, the insulating member 1402 is extrusion-molded to have a gourd-like cross section for reducing a digging of the drain wire 1408 into the insulating member 1402.
Further, in the twinax cable of FIG. 14, the conductor wires 1401, 1404 are commonly covered by the insulating member 1402. In the twinax cable of FIG. 12, although the insulating members 1202, 1205 covering the conductor wires 1201, 1204 are provided in the two insulated electric wires 1203, 1206, the two insulating members 1202, 1205 are not fabricated in the same timing during the manufacturing process (e.g. the two insulating members 1202, 1205 may be formed in different lots). Therefore, dielectric constants of the insulating members 1202, 1205 are not completely equal to each other. On the other hand, in the twinax cable of FIG. 14, all parts of the insulating member 1402 covering the two conductor wires 1401, 1405 are fabricated in the same timing, so that the dielectric constants of a part covering the conductor wire 1401 and a part covering the conductor wire 1405 are equal to each other.
FIG. 15 shows a cross-sectional view of a fourth example of conventional twinax cables, which is disclosed by Japanese Patent Laid-Open No. 2002-289047 (JP-A 2002-289047). As shown in FIG. 15, in the twinax cable of the fourth example, two conductor wires 1501, 1504 are insulated by insulating members 1502, 1505, respectively to provide two insulated electric wires 1503, 1506, and a shield conductor 1507 is wound around the two insulated electric wires 1503, 1506. A drain wire 1508 is lengthwise provided on an outer periphery of the shield conductor 1507 to contact a conducting plane of the shield conductor 1507. The shield conductor 1507 is jacketed with a jacket 1509. The drain wire 1508 is disposed on a side of the insulated electric wire 1503. The drain wire 1508 and the conductor wires 1501, 1504 are pulled out to be parallel with a constant distance at the time of connecting the twinax cable of FIG. 15 to the printed circuit board (as shown in FIG. 16), connection workability is good.
FIG. 16 is a perspective view showing a case of connecting the conventional twinax cable to a printed circuit board by soldering. As shown in FIG. 16, in as state that the twinax cable of FIG. 15 is connected by soldering to a printed circuit board 1606, the two conductor wires 1501, 1504 are connected to signal line pads 1604, 1605 in the printed circuit board 1606, respectively, and the drain wire 1508 is connected to a ground pad (GND pad) 1603. Packaging density of the twinax cable on the printed circuit board 1606 at this time depends upon a width P1 of the jacket 1509 of the twinax cable.
FIG. 17 is a perspective view showing a conventional transmission line using a printed circuit board. As shown in FIG. 17, in the conventional transmission line using the printed circuit board, a signal transmitted from a transceiver IC 1701a is transmitted through a wiring pattern 1709 and via a connector 1707 to a backplane board 1706. A signal transmitted from the backplane board 1706 is transmitted via connector 1704 and through the wiring pattern 1705 to a transceiver IC 1701b which is a receiving terminal. A line card 1703a and a line card 1703b are mated with the connectors 1707 and 1704 to be held by the backplane board 1706.
Common mode noise filters 1708 are in-line provided on the wiring patterns 1709 and 1705, respectively, in order to shut off a common mode component that is the noise. The common mode component arriving at a receiving terminal side is shut off by this common mode noise filter 1708.
However, in the conventional twinax cables, there is a disadvantage of intra skew (i.e. a difference in signal propagation clock time between two conductor wires, hereinafter simply referred to as “skew”).
In the twinax cable of FIG. 12, since there is a gap (i.e. vacant space, air) A in an outer periphery of the drain wire 1208, when the shield conductor 1207 is wound around the drain wire 1208 and the insulating members 1202, 1205, the drain wire 1208 is compressed or displaced, so that the insulating members 1202, 1205 are crushed. As a result, configurations of the twin insulated electric wires 1203, 1206 are asymmetrical. When the configurations of the insulated electric wires 1203, 1206 are asymmetrical in one pair, the twin conductor wires 1201, 1204 are different in propagation constant from each other, so that attenuation characteristic and phase characteristic in the pair of the conductor wires 1201, 1204 are different from each other. This results in generation of the skew. However, it is necessary to reduce the skew so as to transmit the high speed signals of several Gbps or more in the twinax cable.
The skew is generated due to the difference in propagation constant between the twin conductor wires, and three main factors are assumed as immediate causes thereof.
Factor (1): Physical overall lengths of the twin conductor wires are different from each other.
Factor (2): Dielectric constants per se of the insulating members are different from each other in the pair.
Factor (3): The configurations of the insulating member are asymmetrical in the pair, so that effective dielectric constants in the pair are asymmetrical.
Herein, the “dielectric constant” means a parameter showing the dielectric characteristic of the material per se, and the “effective dielectric constant” means an effective dielectric constant in which influences of an electric field leaking into the space is taken into account. In the case that the electric field occurs only inside of a dielectric material (corresponding to the insulating members 1202, 1205 in the twinax cable of FIG. 12, and the insulating member 1402 in the twinax cable of FIG. 14), it is sufficient to consider the dielectric constant. However, since there is the air in vicinity of the dielectric material in an actual twinax cable and the influence of the electric field generated in the air is not negligible, it is necessary to consider the effective dielectric constant. By way of example only, even in the case that the two insulated electric wires 1203, 1206 having the same dielectric constant are prepared, the effective dielectric constants of the respective insulated electric wires 1203, 1206 will be different from each other when the influences affecting on the two insulated electric wires 1203, 1206 are not equal (asymmetrical) due to the cable configuration or manufacturing process for pairing the two insulated electric wires 1203, 1206.
From the view point of the three main factors as described above, the twinax cables of FIG. 13 to FIG. 15 will be contemplated as below.
In the twinax cable of FIG. 13, a stress acting between the insulated electric wires 1303, 1306 and the drain wire 1308 is dispersed, to control the deformation (crush) of the insulating members 1302, 1305, thereby reducing the asymmetry in configurations of the pair of the insulating members 1302, 1305. However, in case that a location of the drain wire 1308 is shifted in a lateral direction in FIG. 13 due to the inaccuracy in manufacturing, a relationship of forces working between the two insulating members 1302, 1305 will be asymmetrical. Accordingly, the deformation condition of the insulated electric wires 1303, 1306 are not completely symmetrical, so that the twinax cable of FIG. 13 does not have a configuration which is rigid against the production tolerance.
Further, in the twinax cable of FIG. 13, electromagnetic coupling between the drain wire 1308 and the conductor wires 1301, 1304 is enhanced by arranging the drain wire 1308 inside of the shield conductor 1307, so that the electric field distribution in the insulating members 1302, 1305 becomes heterogeneous. Accordingly, the current density distribution of electric current flowing through the conductor wires 1301, 1304 is locally varied. As a result, transmission loss (attenuation) increases.
In the twinax cable of FIG. 14, the two conductor wires 1401, 1404 are collectively coated by the single insulating member 1402, thereby reducing a dielectric constant difference in the insulating member generated in the pair. In addition, a characteristic impedance value of the cable is stable since a location of the drain wire 1408 is uniquely determined. However, similarly to the twinax cable of FIG. 13, the drain wire 1408 is disposed inside of the shield conductor 1407, electromagnetic coupling between the drain wire 1408 and the conductor wires 1401, 1404 is locally enhanced, so that the electric field distribution in the insulating member 1402 becomes heterogeneous. Accordingly, the current density distribution of electric current flowing through the conductor wires 1401, 1404 is locally varied. As a result, transmission loss (attenuation) increases.
In the twinax cable of FIG. 15, the drain wire 1508 is disposed outside of the shield conductor 1507, thereby suppressing increase in transmission loss (attenuation). However, it is difficult to produce the twinax cable of FIG. 15 with keeping a location of the drain wire 1508 in a stable state, since it is necessary to arrange the drain wire 1508 having a circular cross section along an arc part of the insulating member 1402. As a result, unstable positioning of the drain wire 1508 causes the deformation of the insulating member 1502, so that the asymmetry of the pair of the insulating members 1502, 1505 easily occurs.
Further, in the twinax cable of FIG. 15, when the location of the drain wire 1508 is shifted, the shield conductor 1507 deforms to be bent inside to fill the gap A. The deformation of the shield conductor 1507 causes turbulence of the electric field distribution in the insulating members 1502, 1505, so that the transmission loss characteristic becomes unstable. Herein, it is difficult to control the deformation degree of the shield conductor 1507 in manufacturing. In other words, the twinax cable of FIG. 15 has a structure in which the asymmetry occurs in the pair of the insulated electric wires in manufacturing. It is similar in the case that the drain wire 1508 is located on a side of the insulated electric wire 1506, oppositely to the example shown in FIG. 15.
As described above, in the twinax cables of FIG. 13 to FIG. 15, the stability to production tolerance is not considered in improving the three main factors as described above. Further, the problems in the three main factors cannot be solved simultaneously. Still further, an effective solution is not proposed for solving the problem of the increase in transmission loss (attenuation).
In addition, when the conventional twinax cable is connected to the printed circuit board, it is necessary to dispose the GND pad 1603 for connecting the drain wire 1508, between one pair of the signal line pads 1604, 1605 and another pair of the signal line pads 1604, 1605, as shown in FIG. 16. On the other hand, the width P1 of the twinax cable is increased by a width of the drain wire 1508. The packaging density cannot be increased, since the packaging density of the twinax cable on the printed circuit board 1606 depends upon the width P1 of the jacket 1509 of the twinax cable. Further, the connection of the printed circuit board 1606 to the GND pad 1603 in FIG. 16 is not easy, when the drain wire 1208 is disposed in a middle of the conductor wires 1201, 1204, such as the twinax cable of FIG. 12.
Still further, in the conventional twinax cable, the common mode noise filter 1708 is indispensable for composing the transmission line, as shown in FIG. 17.